How to Breed Any Monster
You can go to this page to find out how it takes to get a specific monster. The results are sorted in good to bad order, you can also compare different combinations with the "Percentage Bar" - higher, better; or by comparing the "Avg Time" - shorter, better.
Joseph Seeley - MonsterLegendsGuide (MLG) also has the perfect page for this.
How to Breed Legendary Monsters
Basic: Breeding Legendary Monsters is unique process, it doesn't have to be relative between parent monsters and the legendary one. If you have played Dragon City, let's think this process is similar to Deus Vault breeding. Therefore, all you need is having correct parent monsters and some luck!
Joseph Seeley (at monsterlegendsguide.com) and me have been working together to find any report in successfully breeding legend.
Known breedable Legendary monsters
NOTE: Before if you needed 2 specific monsters to get a Legendary monster, for ex: A + B = X, you could use two of the same: A + A = X or B + B = X. Now this is fixed, you can only use one combination A + B = X.